rebase [--onto O] A B: omit needless checkout

This teaches "git rebase [--onto O] A B" to omit an unnecessary checkout
of branch B before it goes on.

"git-rebase" originally was about rebasing the current branch to somewhere
else, and when the extra parameter to name which branch to rebase was
added, it defined the semantics to the safest but stupid "first switch to
the named branch and then operate exactly the same way as if we were
already on that branch".

But the first thing the real part of "rebase" does is to reset the work
tree and the index to the "onto" commit.  Which means the "rebase that
branch" form switched the work tree to the tip of the branch only to
immediately switch again to another commit.  This was wasteful.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -309,22 +309,42 @@ then
 	}
 fi
 
-# If the branch to rebase is given, first switch to it.
+# If the branch to rebase is given, that is the branch we will rebase
+# $branch_name -- branch being rebased, or HEAD (already detached)
+# $orig_head -- commit object name of tip of the branch before rebasing
+# $head_name -- refs/heads/<that-branch> or "detached HEAD"
+switch_to=
 case "$#" in
 2)
+	# Is it "rebase other $branchname" or "rebase other $commit"?
 	branch_name="$2"
-	git-checkout "$2" || usage
+	switch_to="$2"
+
+	if git show-ref --verify --quiet -- "refs/heads/$2" &&
+	   branch=$(git rev-parse --verify "refs/heads/$2" 2>/dev/null)
+	then
+		head_name="refs/heads/$2"
+	elif branch=$(git rev-parse --verify "$2" 2>/dev/null)
+	then
+		head_name="detached HEAD"
+	else
+		usage
+	fi
 	;;
 *)
+	# Do not need to switch branches, we are already on it.
 	if branch_name=`git symbolic-ref -q HEAD`
 	then
+		head_name=$branch_name
 		branch_name=`expr "z$branch_name" : 'zrefs/heads/\(.*\)'`
 	else
+		head_name="detached HEAD"
 		branch_name=HEAD ;# detached
 	fi
+	branch=$(git rev-parse --verify "${branch_name}^0") || exit
 	;;
 esac
-branch=$(git rev-parse --verify "${branch_name}^0") || exit
+orig_head=$branch
 
 # Now we are rebasing commits $upstream..$branch on top of $onto
 
@@ -335,6 +355,8 @@ if test "$upstream" = "$onto" && test "$mb" = "$onto" &&
 	# linear history?
 	! git rev-list --parents "$onto".."$branch" | grep " .* " > /dev/null
 then
+	# Lazily switch to the target branch if needed...
+	test -z "$switch_to" || git checkout "$switch_to"
 	echo >&2 "Current branch $branch_name is up to date."
 	exit 0
 fi
@@ -346,22 +368,11 @@ then
 	GIT_PAGER='' git diff --stat --summary "$mb" "$onto"
 fi
 
-# move to a detached HEAD
+# Detach HEAD and reset the tree
-orig_head=$(git rev-parse HEAD^0)
-head_name=$(git symbolic-ref HEAD 2> /dev/null)
-case "$head_name" in
-'')
-	head_name="detached HEAD"
-	;;
-*)
-	git checkout "$orig_head" > /dev/null 2>&1 ||
-		die "could not detach HEAD"
-	;;
-esac
-
-# Rewind the head to "$onto"; this saves our current head in ORIG_HEAD.
 echo "First, rewinding head to replay your work on top of it..."
-git-reset --hard "$onto"
+git checkout "$onto^0" >/dev/null 2>&1 ||
+	die "could not detach HEAD"
+# git reset --hard "$onto^0"
 
 # If the $onto is a proper descendant of the tip of the branch, then
 # we just fast forwarded.
@@ -374,7 +385,8 @@ fi
 
 if test -z "$do_merge"
 then
-	git format-patch -k --stdout --full-index --ignore-if-in-upstream "$upstream"..ORIG_HEAD |
+	git format-patch -k --stdout --full-index --ignore-if-in-upstream \
+		"$upstream..$orig_head" |
 	git am $git_am_opt --rebasing --resolvemsg="$RESOLVEMSG" &&
 	move_to_original_branch
 	ret=$?
@@ -397,7 +409,7 @@ echo "$orig_head" > "$dotest/orig-head"
 echo "$head_name" > "$dotest/head-name"
 
 msgnum=0
-for cmt in `git rev-list --reverse --no-merges "$upstream"..ORIG_HEAD`
+for cmt in `git rev-list --reverse --no-merges "$upstream..$orig_head"`
 do
 	msgnum=$(($msgnum + 1))
 	echo "$cmt" > "$dotest/cmt.$msgnum"